What is the overhead cost of an empty vector?

Question

What is the memory overhead of having an empty vector vs having a pointer to a vector?

Option A:

std::vector<int> v;

Option B:

std::vector<int> *v = NULL;

I believe that option B takes 1 32 bit pointer (assuming 32 bit here) How much memory does the empty 'v' take up?

Answer 1

Implementation dependant, probably a pointer and two integers for current size and capacity.

Answer 2

std::vector v; takes up sizeof(v) space. It might vary by implementation, so run it and find out how much it takes for you.

Answer 3

Firstly there's no such thing as a "std::vector v". Vector is a template-based collection and thus needs a type. E.g

std::vector<int> v;

Secondly it's completely implementation-dependent and you should neither assume nor rely on the details. For what it's worth it's 20-bytes using VC.

Answer 4

As for the question as asked: It depends on the implementation. With MSVC 7.1 this:

std:: cout << sizeof(std::vector<int>) << std::endl;

gives me 16. (3 pointers: begin, end, and end of capacity, plus an allocator)

However it should be noted that the pointer-to-vector has a larger overhead:

• in both time and space in the non-empty case
• in complexity in all cases.

Answer 5

VS2005:

std::vector<int> *ptrToVec = new std::vector<int>();
std::vector<int> vecOfInt;

sizeof(ptrToVec) = 4
sizeof(vecOfInt) = 20

Thanks!